FAQ

(07/01/2010)

The FLOW CONSULTANT^™ license applies to all FAQ.

1.   What is the difference between a clamped plate and a freely supported plate selection?

 Ans:  Orifice plates are installed (mounted) by either “Clamping” between flanges or “Freely supported” as in an orifice fitting. The plate bends due to differential pressure, bending being greater for a freely supported plate then for a clamped plate.

 The discharge coefficient of a standardized orifice plate changes with plate deflection (or bending). The maximum deflection is limited to 0.005(D-d)/2 to maintain the standardized estimated discharge coefficient value.

 Based on differential pressure across the plate The FLOW CONSULTANT uses the Roark stress equations to calculate deflection for either a clamped or fitting mounted plate and calculates expected bias.

 For Restrictive orifices the standardized limit requirement is used to compute (via Roark) minimum plate thickness, which, because of the high differential, makes plate thickness larger than standardized plates.

It is important to note that The FLOW CONSULTANT calculates maximum differential across the plate, which is not the overall pressure loss entered for the overall restrictive pressure loss (entered differential). Calculated stress (Roark) is then based on actual differential across the plate, which is higher than the overall pressure loss. 

2.   Version 7 has two selections for limiting the flow of a liquid by sizing an orifice such that cavitation occurs, what are the differences between them?

Ans:  A fully Cavitating orifice limits liquid flow by creating a “Cavitation Zone” located at the minimum flow area, resulting in a “boiling” liquid due to low pressure, rather than boiling at the liquid’s  saturation temperature.

 This zone negates Bernoulli’s equation and differential is fixed as the difference between upstream pressure and liquid vapor pressure for the flowing temperature. The FLOW CONSULTANT enters this differential and the user cannot change it for all the available fluid selections (Water, Propane, Butane, etc.). It can, however, be changed if fluid properties are “ENTERED” and the vapor pressure value entered.

Cavitation bubbles implode as pressure recovers along the pipe, resulting in a highly destructive mechanism that can destroy the inner pipe wall. To minimize, or eliminate, destruction a standard Venturi design is recommended. However, many users select a standard orifice or long cylinder. The Venturi or cylinder develops a thin liquid protection layer (boundary layer) that negates the effect of implosion.

The V7.mdb has two examples

For a thick orifice                                                                        Example Cavitation:Thick Orifi   

For a standard ASME VenturiV7.mdb Miller Example 13.3       cavitating     ASME Venturi Machined inlet (Example 13.3 is from Miller, 3^rdEdition)

3.   Please be informed that, when I have to use the flow rate expressed in MMSCFD, the field to insert the reference condition doesn’t appear; please see below

Ans.   You have selected a Vapor Flow (see upper left hand corner). By definition a vapor is a gas that exists as a liquid at room temperature and pressure. Therefore, a standard volume (MMSCH) is impossible since the fluid is not a gas at a base temperature and pressure. (60⁰F, 14.7 psia).

Simply select a gas rather than a vapor (or a liquid).

The following self explanatory message also appears if you wanted to provide a Specific Gravity or Molecular Weight input.

4. How do I confirm the AGA-8 program computed compressibility?

Ans.  AGA-8 presents a list of gas composition from various regions of the world (Table A.5-1).  These compositions, in percent mole fractions, are entered into the FLOW CONSULTANT input screen. When selecting a pressure and temperature for that gas composition,  the calculated compressibility (Z_f) is checked against the AGA-8 Table A.5-2.

                                     50⁰F     1200 psia                       Z_f = 0.816426

Table Z_f=0.816427    @50⁰F, 1200 psia

Flow Consultant       Z_f = 0.816426

Important Note:  The AGA-8 equation of state is written in terms of molar density

The FLOW CONSULTANT uses this equation to compute both flowing and base density. The compressibility factor is shown only for information purposes. The exact mass flow rate and standard volume flow is obtained by the generic solution presented in Miller, The Flow Measurement Engineering Handbook, and 3rd Edition pp 9.107-9.117. The equation for standard (or normal) volumes is      

This computation is exact since compressibility is derived from the molar density.

5. The FLOW CONSULTANT uniquely covers restrictive and critical flow meters. While restrictive orifices and liquid Cavitating limiting flow-rate devices are not flow meters they are widely used to limit or provide a pressure loss (restriction) in a flow system.

Ans.  The difference between limiting flow and restricting flow are often misunderstood.

Critical (gas/vapor) or Cavitating Liquid Flow:  Gas Flow-rate is limited to a fixed flow-rate for a given upstream pressure when a shock front is created at the minimum flow area of the device, this is established when the “Critical” pressure ratio is reached as the downstream pressure is reduced. For air this occurs when the ratio of upstream pressure to throat pressure is 0.53.

Liquid flow is limited when the differential pressure equals the difference between upstream and throat (or for an orifice) vena contracta pressure is the vapor pressure of the fluid.

In both cases Bernoulli’s no longer applies.  For a thick orifice the overall pressure loss the difference between the upstream pressure and the downstream pressure. For a standardized critical flowmeter the pressure loss is approximately 10 % of the choking difference between upstream and downstream choking pressure.

Version 7 includes critical flow thick orifices, Torodial Throat and Cylindrical Venturi.

Restrictive Orifices:  Restrictive orifices do not limit the flow but fix overall pressure loss for a given flow rate. The FLOW CONSULTANT computes the pressure loss using the standard orifice equation with the Stolz derived discharge coefficient for pipe taps.

Pipe tap are located 2.5 pipe diameters upstream of the plate and 8 pipe diameters downstream (2 1/2 and 8 D taps). This prediction equation well fits recent overall pressure loss data (Miller p/ 6.41).

For gases a thin orifice plate does not choke at the critical pressure ratio (see Figure below). The gas expansion factor has a defined “bend” at the critical pressure ratio (Miller, p 9.20).  Cunningham’s equation for the gas expansion factor is used in the flow consultant in this range.

Overall pressure loss for a restrictive plate, used for sizing, is not the same as the pressure drop across the plate.  The FLOW CONSULTANT computes the differential based on the corner tap pressure, as this is the actual pressure difference across the plate. It is this differential that is used in the Roark stress/strain equations to determine plate thickness.

Multiple Plates: Often a single orifice will not drop the required pressure at the desired flow-rate. In this case multiple orifices are required. There is no data for multiple plate spacing, however, it is reasonable to assume pressure has 90 to 95% recovered after the eight (8) pipe diameters and this be used as a spacing requirement between plates.

Estimated uncertainty: It is estimated, based on the computational

6. What the differences are between the ASME-3M, ISO 5167, and ANSI 2530, API Chapter 14 orifice standards.

Ans. Aside from slight difference in minor areas a comparison shows

·        ASME-3M and ISO 5167 are identical in all area. ASME-3M, however, uses US units.

·        AGA-3, ANSI 2530 and API Chapter 14 are identical.

A comparison between ANSI 2530 and ISO 5167 shows the major differences as

·        The ISO 5167 discharge coefficient equation is based on both liquid and gas data while the ANSI 2530 does not. Therefore, results are different between standards.

·        ISO 5167 has a different orifice gas expansion factor that replaces the 1935 developed equation, ANSI 2530 retains the original equation. A significant difference occurs for high ratios of differential to flowing pressure.

·        ANSI 2530 flow-rate equations use a differential pressure unit based on 60 ⁰ F water, ASME-3M uses 68 ⁰ F, and ISO 5167 does not use water differential pressure unit. The differential pressure transmitter must be calibrated to the correct temperature reference or a 0.05 % flow-rate calculation bias error occurs.